[codex] add std mutation testing

[greenthree]

What changed

• Add probhub mutation and probhub mutate for conservative C++ standard-solution mutation testing.
• Add comparison-boundary, boolean-negation, and integer-boundary operators with masked comments/literals, stable IDs, plan hashes, and bounded evidence.
• Run each mutant from an isolated snapshot through the existing Validator/Judge/process control; formal sources, PDFs, ZIPs, metadata, and Manifests are never modified.
• Integrate mutation state and kill/survivor summaries into reports, Skill guidance, CLI reference, README, CHANGELOG, and a committed mutation Fixture.
• Preserve old evidence on cancellation, timeout, infrastructure failure, input changes, evidence-size overflow, lock contention, or publish failure.
• Keep accepted sources at their original relative path in snapshots and forward the remaining overall deadline to each Judge invocation.

Why

Mutation testing supplements expected-matrix, independent-solution, and stress evidence by exposing boundary changes that current data does not distinguish. It is intentionally not a proof of correctness and is not a build hard gate.

Validation

• npm run check: 678 passed, 30 skipped
• npm run pack:check: probhub=220, probhub-skill=5
• Skill quick validation passed
• python -m py_compile probhub\\mutation.py probhub\\cli.py probhub\\reporting.py passed
• python -m unittest tests.test_mutation tests.test_report tests.test_workspace_fixtures.FixtureCatalogTests -v: 23 passed
• Current Core against phtest: all nine problems linted successfully; phtest remains clean and unpushed
• git diff --check passed